1. Field of the Invention
The present invention relates to a technique of thermally refining a porous coal having a high oxygen content. More particularly, the invention relates to a technique of thermally refining low rank porous coal, which is considered to be of low economic value due to its high water content, by effectively dewatering the coal and allowing an oil to be adsorbed onto the pore surface of the coal to eliminate the risk of spontaneous combustion of the coal, and also by decarboxylating and chemically dehydrating the coal to decompose or to release oxygen-containing groups such as carboxy or hydroxy of the raw coal to reduce the porosity of the porous coal.
2. Description of the Related Art
Porous coal tends to contain a considerable amount, for example 30 to 70% by weight of water depending on its porosity. If the porous coal of such a high water content is to be transported, for example, to industrial area, it requires a relatively high transportation cost as if water itself were transported, so that the it is only viable to use porous coal near coal fields. Therefore, it has been accepted that porous coal cannot be utilized other than in the vicinity of the coal field. A typical example of porous coal having a high water content is brown coal.
Although certain brown coals have favorable characteristics such as having low ash and sulfur contents, they tend to have a higher water content because of their porosity. If the water content exceeds 30% by weight, the transportation costs increase considerably, and calorific value decreases commensurate with the higher water content, or higher oxygen content in the dry state. Therefore, brown coals are categorized as low rank coals, notwithstanding the above-mentioned favorable characteristics. This is a problem not only with brown coals, but also with lignite and sub-bituminous coal. Although a description will be given taking brown coals as an example in this specification, it should be borne in mind that the present invention is applicable to any porous coals including lignite and sub-bituminous coal. In addition, the invention is applicable to any brown coals including Victorian coal, North Dakota coal, Beluga coal, etc., irrespective of their production districts as long as they are porous and have a high water content.
In light of decreasing energy resources, techniques for effectively utilizing brown coal have been studied. Thermal refining of brown coal is known as one such technique. This technique is advantageous in that spontaneous combustion is inhibited since the pores of the coal shrink as a decarboxylating/dehydrating reaction proceeds to expel water. However, because raw brown coal containing a great amount of water is treated with heat in the thermal refining process, it is necessary that the heating process must be kept above water vapor pressure which is very high. Moreover, since the dewatering process involves a pyrosis reaction, the waste water discharged therefrom contains a number of organic components which increase the burden of waste water treatment. Therefore, a practical technique for utilizing porous coals by thermal treatment is yet to be realized.